1. Field of the Invention
The present invention relates to a film formation apparatus and method for a semiconductor process for forming a thin film doped with an impurity (dopant), such as phosphorous (P) or boron (B), on a target substrate, such as a semiconductor wafer. The term “semiconductor process” used herein includes various kinds of processes which are performed to manufacture a semiconductor device or a structure having wiring layers, electrodes, and the like to be connected to a semiconductor device, on a target substrate, such as a semiconductor wafer or a glass substrate used for an LCD (Liquid Crystal Display) or FPD (Flat Panel Display), by forming semiconductor layers, insulating layers, and conductive layers in predetermined patterns on the target substrate.
2. Description of the Related Art
In manufacturing semiconductor devices for constituting semiconductor integrated circuits, a target substrate, such as a semiconductor wafer, is subjected to various processes, such as film formation, oxidation, diffusion, reformation, annealing, and etching. As CVD (Chemical Vapor Deposition) used as a film formation process, there is a method of simultaneously supplying a source gas for film formation and a doping gas for doping the deposition film with an impurity. Jpn. Pat. Appln. KOKAI Publication No. 2003-282566 discloses a CVD method of this kind performed in a vertical heat processing apparatus. According to this method, a number of semiconductor wafers are accommodated at intervals in the vertical direction within a vertical process container. Then, a film formation gas and a doping gas containing an impurity are supplied into the process container while the wafers are being heated. As a consequence, a thin film is deposited on the wafers, while being doped with the impurity. For example, in a process where poly-crystalline silicon doped with phosphorous is deposited, PH3 gas is used as a doping gas.
Where a doping gas of this kind is a material having a high vapor pressure, the doping gas can be supplied in a pure state from a storage tank into a process container at a controlled flow rate. However, in general, doping gases of this kind have a very low vapor pressure. Accordingly, where a doping gas in a pure state is supplied into a process container, it cannot diffuse sufficiently, thereby resulting in a less uniform doping distribution.
For this reason, in general, where a doping gas of this kind is supplied, this doping gas is diluted in advance to, e.g., about 1% by an inactive gas, such as N2, and is stored in a storage cylinder. When used, this diluted 1% doping gas is discharged from the storage cylinder at a controlled flow rate, and is supplied into a process container with a high diffusion rate. In this case, i.e., where the diluted doping gas is supplied from the storage cylinder into the process container, the gas flow rate per unit time becomes larger by that much corresponding to dilution. As a consequence, the doping gas can swiftly and uniformly diffuse in a short time within the process container, which has a relatively large volume.
However, in this case, as described above, the consumption (outflow) per unit time of the diluted doping gas from the storage cylinder is very high. As a consequence, the storage cylinder needs to be replaced with a new one in a relatively short time, thereby resulting in a low productivity and thus a low throughput. Particularly, as the wafer size increases from 8 inches to 12 inches (300 mm), the volume of batch-type process containers greatly increases. Under the circumstances, it has become more necessary for a doping gas being supplied to swiftly and uniformly diffuse within a process container while maintaining high throughput of the process.